I heard Chapter 12 introduces a new story. Can you tell me who the story is about and what background information is provided?

You are correct, Chapter 12 sets the stage for a new narrative. The text describes it as a "very delightful story" meant to illustrate the greatness of the Guru. The main character introduced is a ritualistic, orthodox Agnihotri (a keeper of the sacred fire) from the holy city of Nashik, whose surname is Mule. The chapter explains that his arrival in Shirdi was not a simple matter but occurred due to the "strength of his past merits." It emphasizes this point by stating that without such merit, a person cannot stay in Shirdi for even a moment. This introduction builds anticipation for the story of how this devout man had a rare and fated meeting with Sai.